Окно отображается по-разному до и после запуска. В чём причина?

В основном Grid у меня находятся два Grid'а. На одном разметил сетку (через Border), на втором график. Я знаю, что можно сделать это в одном гриде, но для восприятия мне так проще. Точки графика устанавливал вручную,ориентируясь на свою же сетку с другого Grid. До запуска приложения всё выглядит ровно:

После запуска всё съезжает. Из-за чего так и как исправить?

Код:

        Title="MainWindow" Height="560" Width="750" WindowStartupLocation="CenterScreen">

<Grid>
    <!--Сетка-->
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="60"/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Border Grid.Column="0" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="1" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="2" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="3" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="4" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="5" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="6" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="7" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="8" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="9" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="10" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="11" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="12" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="13" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="14" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="15" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="16" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="17" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="18" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="19" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="20" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="21" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="22" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="23" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="24" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="25" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="26" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
        <Border Grid.Column="27" BorderThickness="0,0,2,0" BorderBrush="#b1dcd2"/>
    </Grid>
    <!--График-->
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="40"/>
            <RowDefinition Height="40"/>
            <RowDefinition />
            <RowDefinition />
            <RowDefinition />
            <RowDefinition />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="58"/>
            <ColumnDefinition/>
            <ColumnDefinition Width="320"/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <!--NRZ график-->
        <Polyline Grid.Row="2" Grid.Column="1" Grid.ColumnSpan="3" Points="0,69, 25.65,69, 25.65,25.65, 50.3,25.65, 50.3,69, 74.95,69, 74.95,25.65, 99.6,25.65, 99.6,69, 124.25,69, 124.25, 25.65, 148.9,25.65, 148.9,69, 247.5,69, 247.5,25.65, 494,25.65, 494, 69, 666.55, 69, 666.55,25.65" Stroke="Black" StrokeThickness="4"/>
    </Grid>
</Grid>